1. NEUTRAL HYDROGEN Frank Briggs RSAA and ATNF z = 8 z = 0

2. Neutral gas in nearby GALAXIES Optical + Radio

3. Neutral gas in nearby GALAXIES Radio only Australia Telescope Compact Array

4. Neutral gas in nearby GALAXIES Velocity field from 21cm Line Rotation Curve

5. Neutral gas in nearby GALAXIES Velocity field from 21cm Line Rotation Curve first strong evidence for Dark Matter in galaxies Important Kinematic Tracer

9. The “Mice”

10. our view of the Milky Way in the 21cm line

11. HI Blue galaxies young stars Recent Star Formation the fuel for

12. HI = fuel for star formation …. galaxies HI = kinematic tracer …. deduce galaxy mass distributions

13. Big Bang Nucleosynthesis.…. ionized…… CMB

14. damped Lyman-  abs-lines 21cm Line Studies (HIPASS)

15.  M - 0.5  Tot 0 1/3300 looks like HI is an endangered species !!!

16. Where are the missing baryons ???

17. ??? How do you find H clouds ??? ??? How do you find HI clouds ??? n HI n HI +n HII  1 1 If neutral fraction

18. Energy Levels in Hydrogen E = 0 Ground State Energy Lyman Balmer Paschen

19. Emission Lines: from Photoionization Lyman alpha

20. Energy Levels in Hydrogen E = 0 Ground State Energy free electron Photo- ionization Recombination radiative decay

21. Energy Levels in Hydrogen E = 0 Ground State Energy free electron Photo- ionization Recombination radiative decay free electron Lose Kinetic Energy!

22. Energy Levels in Hydrogen E = 0 Ground State Energy To excite optical & UV lines …need at least 10.2 eV Hot Stars Quasars

23. Energy Levels in Hydrogen E = 0 Ground State Energy Hyperfine splitting of H ground state 21 cm line  E = 6 x 10 eV -6-6 thermally excited by collisions in cold gas

24. Recombination competes with Re-ionization

25. Time scales for Recombination … t = n /R recomb e # Electrons /cm 3 # Recombinations /sec /cm 3 = n /(n n  ) eep recomb

26. Time scales for Recombination in IGM… t = n /R recomb e = n /(n n  ) eep recomb = 1 /( n  ) p recomb = T /( n 2 x 10  ) sec p 1/2 -11 but, n ~ n proportional ~ (1+z) 3 p baryon t proportional T /(1+z) recomb 3 1/2

27. Time scales for Recombination … t = n /R recomb e = n /(n n  ) eep recomb t proportional T /(1+z) recomb 3 1/2 at z=9 t ~ 10 yrs recomb 8 at z=0 t ~ 10 yrs recomb 11

28. i.e.  Sea of ionized baryons StarsHI gas Stars & HI gas ?? Just HI ?? Cartoon for Z = 0

29. Is there an invisible population of LSB but HI rich clouds ??? How could it escape notice ???

30. Radio spectroscopy

31. Absolute Magnitude Optical Luminosity Function + info on HI Content of galaxies … (2df Madgwick etal)

32. HI Mass Function: (dashed curve)

33. HI Content according to galaxy mass

34. “Blind” surveys for HI clouds… Arecibo Telescope

35. Feed Systems

36. Arecibo Telescope … very good for looking straight up !

37. Frequency Time

39. Arecibo Survey results 65 detections (Zwaan et al 1997) Log( M /M ) HI solar

40. Arecibo: HI content according to galaxy mass (Zwaan et al 1997)

41. where are HI selected galaxies located ??? Follow the same Large Scale Structure as optically selected… … except in rich clusters, which are HI deficient

42. Latest (and greatest) survey: HIPASS Consortium: ATNF, Melbourne, Cardiff, RSAA +

43. Latest (and greatest) survey: HIPASS Results: detected 1000’s of galaxies… no clear IG HI Clouds

45. Summary of HI in nearby (z~0) Universe: good tracer of galaxies but, only a tiny fraction of total baryons are neutral looks like hydrogen must be confined, so that it can shield itself against the ionizing background if confined to sufficient density to remain neutral, the gas is unstable to collapse and star formation => at z=0, HI is inevitably associated with stars